Vegetation of the Exe Estuary in Relation to Water Salinity

Abstract

Upriver zonation of angiosperms is controlled largely by water salinity but plant competition and mobility of substrate are important. Some species of marine algae have a wide range of salt-tolerance and penetrate upstream into freshwater reaches. No freshwater algae tolerated more than a trace of salt. Type of substratum as well as salinity has an important bearing on algal distribution. Diurnal fluctuations of salt content are plotted for various parts of the estuary and the salinity gradient at high tide traced from freshwater conditions to the sea. The soil of the upper littoral zone frequently shows a higher salt concentration than that further downshore as water salinity may be fairly small at low tide. Water pH in midstream is unrelated to salinity; that of water taken from river margins and of soil solution in the intertidal zone show a marked correlation; alkalinity increasing with increasing salinity. Abrupt constrictions of the channel hinder upstream penetration of salt water and have a profound effect on plant distribution. Upstream zonation of the principal marginal communities is Zosteretum, Spartinetum, Scirpetum, Phragmitetum and Phalaretum with considerable overlapping of the first two. Comparisons are drawn with vegetation and salinity of tributaries of the Exe and adjacent estuaries of the Teign and Otter.